This invention relates to apparatus used in boreholes, and more particularly to apparatus for detecting the free point of a conduit in a borehole.
When a conduit, such as a drill string, becomes stuck in a borehole, a conventional method for determining the depth of the free point is to apply torsions and tensions to the conduit from the surface and then determine to what depth these deformations are transmitted. To detect these deformations, an apparatus is lowered into the conduit at the end of a cable and placed at successive depths.
A conventional free-point detection apparatus, described for example in U.S. Pat. No. 3,686,943 (W. D. Smith), comprises a body member having an upper part and a lower part mounted for limited movements with respect to each other, and upper and lower anchoring elements mounted respectively on the upper and lower parts to simultaneously immobilize each part in two longitudinally spaced zones of the conduit. Electric motors driven via the cable move the anchoring elements away from and toward the body member, and a transducer mounted between the parts of the body member detects the relative movements of the body parts when the conduit is deformed elastically by stresses applied from the surface.
Most known transducers must be reset before each measurement. With those transducers which have a substantially linear response, an approximate resetting is sufficient. It is then sufficient that the transducer be brought to an initial position allowing relative rotation and elongation between the upper and lower parts of the apparatus. Such transducers offer definite advantages but also exhibit drawbacks particular to each type used.
The transducer described in the above '943 patent has the disadvantage that it does not differentiate the longitudinal movements from the rotational movements. It is in fact desirable in certain cases to be able to determine whether a torsion applied to the drill pipes from the surface has been transmitted downhole. In particular, when one wishes to unscrew the free part of the pipes, it is necessary to apply an unscrewing torque to a particular joint placed slightly under tension before exploding a charge at the level of this joint. This operation, known as backing off, is quite common. In deviated wells having a bend, the torque applied to the drill pipes from the surface is transmitted poorly downhole, and it is therefore current practice to pull and release the drill pipes at the same time that the torque is applied in order to overcome the friction along the borehole. A transducer of the type described in the '943 patent does not make it possible to determine whether the torque has been transmitted down the hole, because the output signal is indistinguishably influenced by both the tensions and the torsions.
Another transducer, described in U.S. Pat. No. 4,105,071 (Y. Nicolas & A. Landaud) delivers two output signals, one independent of the tensions and the other independent of the torsions applied to the drill pipes. This transducer comprises two distinct parts, one of which is deformed by the tension forces and the other by the torsion forces. Such a transducer has a significant stiffness, and high forces are therefore necessary for deforming it. Under these conditions, it often occurs that the anchors for the apparatus are not sufficiently powerful to transmit such forces, but slip within the drill pipes without deforming the sensitive components of the transducer.